1. Field of the Invention
The present invention relates to a display device and a manufacturing method thereof. In addition, the invention relates to a television set using the display device.
2. Description of the Related Art
In recent years, a flat panel display (FPD) is attracting attention as an alternative display device for a conventional CRT display device. In particular, development of a large-screen television set mounted with a large display panel which is driven with an active matrix method is the primary task for panel manufacturers.
In the conventional display device, a semiconductor element for driving each pixel is formed of a thin film transistor (hereinafter also referred to as a TFT) which uses amorphous silicon as its active layer (see Patent Document 1).
Meanwhile, in the conventional liquid crystal television set, there has been a defect in that images are blurred due to the limitation of the viewing angle characteristics, limitation of the high speed operation with liquid crystal materials, and the like. However, in recent years, there is proposed a new display mode for solving such a problem, which is an OCB (Optically Compensated Bend) mode (see Non-patent Document 1).
[Patent Document 1] Japanese Patent Laid-Open No. Hei 5-35207
[Non-patent Document 1] Nikkei Microdevices' Flat Panel Display 2002 Yearbook, edited by Yasuhiro Nagahiro and others, Nikkei Business Publications, Inc., published in October, 2001 (pp. 102-109)
However, when a TFT formed of an amorphous silicon (amorphous semiconductor) film is driven with direct current, threshold voltage thereof easily varies, resulting in variations of the TFT characteristics. Therefore, in the display device using such a TFT formed of an amorphous semiconductor film for switching of the pixel, luminance unevenness occurs. Such a phenomenon is recognized more often in a large-screen television set having an opposite angle of 30 inches or more (typically, 40 inches or more), which thus poses a serious problem regarding the degradation in image quality.
Meanwhile, in order to enhance the image quality, a switching element capable of high-speed operation is required. However, the TFT using an amorphous semiconductor film has a limitation in operation speed. For example, it is difficult to realize a liquid crystal display device in the OCB mode.